The presence of significant quantities of H.sub.2 S and CO.sub.2 in various "sour" industrial gaseous streams poses a persistent problem. Although various procedures have been developed to remove and recover these contaminants, most such processes are deficient, for a variety of reasons.
In one cyclic method currently attracting attention, the sour gas is contacted, preferably with a solvent-reactant system which comprises a regenerable reactant, to produce solid free sulfur which is recovered either prior or subsequent to regeneration. Suitable reactant materials include polyvalent metallic ions, such as iron, vanadium, copper, manganese, and nickel, and include polyvalent metal chelates. Preferred reactants are coordination complexes in which the polyvalent metals form chelates with specified organic acids.
In yet another process, e.g., that disclosed in U.S. Pat. No. 4,091,073, issued May 23, 1978, to Winkler, CO.sub.2 present in the gaseous stream is also removed by the use of a suitable selective absorbent.
Other processes, such as that described in U.S. Pat. No. 3,226,320 to Meuly, involve removal of impurities from aqueous streams utilizing various polyvalent metal chelates.
Because these "cleanup" processes generally represent significant costs to manufacturing operations, any improvements in such processes which increase their efficiency may have great economic importance. For example, where ligands or chelates or polyvalent metals are employed, degradation or decomposition of the polyvalent metals represents an important cost in the process, as well as requiring measures for decomposition product bleed, removal or treatment, and addition of fresh solution. Even in the case of preferred chelates such as those of N-(2-hydroxyethyl) ethylene diamine triacetic acid and nitrilotriacetic acid, ligand decomposition, over a period of time, requires attention to prevent build-up of decomposition products and consequent loss of efficiency. As will be recognized, the bleed from such processes contains, along with the decomposition products, a considerable amount of the valuable chelate or chelates. Application Ser. No. 441,830, filed Nov. 15, 1982, and application Ser. No. 452,342, filed Dec. 22, 1982, describe processes for recovery of nitrilotriacetic acid from aqueous solutions. However, the volumes of solution to be handled in the processes mentioned may be quite large. A process that reduced the volumes of solution treated would be a decided advantage.
As noted in the aforementioned disclosures, oxalate ion is present in the solutions employed, apparently as a decomposition product of the ligands employed. While the presence of limited quantities of oxalate ion appears beneficial (see, e.g., U.S. Pat. No. 4,009,251), in practice, the concentration of oxalate ion is significant in determination of bleed rate of the solution. Accordingly, selective removal of oxalate ion from the ligand solution would tend to minimize bleed rate and improve the economics of such processes. The invention provides for such removal.